Summer, possibly the most widely discussed and vigorously denounced Washington season, begins today, with temperatures expected to reach the 90s for the fifth time in eight days.

Summer officially begins at 7:45 a.m., according to the National Weather Service.

Summer's first day will be "fairly typical," said wether service forecaster Harold Hess.

He said it would be "warm, probably on the humid side," with more sunshine likely and a possibly of later afternoon thunderstorms. The temperature could reach 93 degrees.

On Monday, the first full day of the season, temperatures in the 90s are again expected. Wednesday and Thursday should be cooler, according to the weather service.

Yesterday, the last full day of spring, thunderstorms swept acorss much of the eastern seaboard, from Long Island, N.Y., to; Virginia's Tidewater area, and brief, heavy downpours soaked much of the Washington area during the morning and afternoon.

Lightning knocked out power to about 2,000 customers of the Virginia Electric and Power Co. in North Arlington and McLean between 3 p.m. and 4:30.

Hagerstown, Md., reported about 1.3 inches of rain yesterday, and almost an inch fell in Frederick, Md.

Leesburg, Va., recorded about an inch in a one-hour period early yesterday morning. In Washington, the figure was .37 inches.

Although humidty was high, temperatures here did not climb above 84 degrees, failing to reach the 88 degree figure that is normal for this time of year.

Last week's 90-degree readings included 91 on Sunday, June 14; 97 on Monday; 96 on Tuesday and 90 on Friday.

Washingtonians did not respond to summer's arrival in any extraordinary way.

Traffic this weekend on the Chesapeake Bay Bridge, a baronmeter of Washingtonians' eagerness to flee the sweltering city for the cool breezes of Atlantic Ocean beaches, was described as average by police at the bridge.

Cpl. Ronnie Cook estimated about 130,000 vehicles would have used the bridge by the time the weekend ended.

The bridge leads to Ocean City, Md., and the size of the crowd at the resort town was also described by police officer Brad Belden as about "average."

In addition to being the first day of summer, today, according to astronomers, will also be the longest day of the year, with 14 hours and 54 minutes elapsing between Washington's 5:43 a.m. sunrise and the city's 8:37 p.m. sunset.

The longest day and the start of summer come when the sun reaches the northernmost point of its annual journey between the tropics of Cancer and Capricorn.

The next few days will be almost as long as today. Then the period between sunrise and sunset will diminish, gradually at first, then at an incresing rate, until the sun crosses the equator and autumn arrives on Sept. 22, a date with equal periods of daylight and darkness.